In manufacturing photovoltaic cells, in particular, layers of amorphous silicon currently are used as active photovoltaic layers. One problem with such layers is that they degrade when exposed to light. This degradation is known as Straebler-Wronski degradation. The output of solar cells using this material deteriorates quickly.
Experiments using microcrystalline silicon, also called nanocrystalline silicon, in such solar cells nearly always yielded poor results. The high rate of defects in this type of layer effectively prohibits its use, and only a portion of the microcrystalline silicon layer participates in collecting charge conductors. Thus, the resulting photocurrent is weak and the cell is not very useful. These flaws have been attributed to the material itself, which is often considered useless.
Certain studies published in Appl. Phys. Lett. 65(7), p. 860, Aug. 15, 1994 entitled "Complete Microcrystalline p-i-n Solar Cell--Crystalline or Amorphous Cell Behavior?" by M. M. Meier, Fluckiger, Keppner and Shah, demonstrate an involuntary doping phenomena. Depositing a layer of silicon using a conventional method such as vapor deposition known as "CVD" (Chemical Vapor Deposition) or plasma deposition in the presence of silane results in a slightly negatively-doped layer. This negative doping results in a layer that is of little or no use in a photovoltaic cell.
Experiments have been performed with a deposition method which would eliminate this negative doping. They consist of adding to the deposition gas a gas that produces positive doping to compensate for the involuntary negative doping of the microcrystalline silicon layer. This method is described in the document "IEEE 1994," p. 409-412, entitled "Intrinsic Microcrystalline Silicon--A Promising New Thin Film Solar Cell Material" by M. M. Meier, Dubail., Fluckiger, Fischer, Keppner and Shah. While it produces particularly interesting results, it is difficult to apply in industry. The amount of doping gas to be introduced into the deposition chamber depends upon a certain number of parameters that are difficult to master. These parameters consist of the quantity of desorption of gas in the deposition chamber, the flux speed of the gas, and the speed at which the layer is deposited. This method has demonstrated the feasibility of a layer using this type of material. It also demonstrates the interest that such a layer has for manufacturers of solar cells but, because of difficulties encountered in controlling the deposition parameters, there has been no industrial application of the method and it remains uniquely a laboratory procedure.